Thoracic aortic aneurysm and dissection (TAAD) are the most deadly manifestation of the inherited connective tissue disorder, Marfan syndrome (MFS). MFS is caused by mutations in the fibrillin-1 gene (FBN-1). Although the molecular and cellular mechanisms driving TAAD remain unclear, transforming growth factor-beta (TGF-?) has become a major target. However, the most recent data from the largest randomized clinical trial showed that targeting TGF-? signaling by losartan was not effective in controlling aortic dilatation. In our preliminary studies, the morphological changes of the aorta appeared earlier than increased TGF-? activation in mgR/mgR mice, a widely used murine model of MFS, indicating that the early pathological changes were independent of enhanced TGF-? activation. Aortic smooth muscle cells (SMCs) play a pivotal role in vasculogenesis. Unexpectedly, we discovered that the aortic SMCs in MFS mice at PD7 are in the contractile phenotype. Since aortic SMCs in wild type mice are normally in the synthetic phenotype at this stage, our findings indicate a premature phenotypic switch of the SMCs to a contractile phenotype in MFS mice. This phenotypic switch during the critical time in the aortic development would be expected to have a dramatic effect on aortic integrity. The contractile phenotype seen in the mgR/mgR mice is associated with a concurrent decrease in Krppel-like factor 4 (KLF4) expression. KLF4 is a DNA-binding transcription regulator that drives phenotypic modulation of aortic SMCs. Importantly, we also found that early treatment with cyclosporine A (CsA) increased KLF4 expression in aortic SMCs and improved aortic morphology in mgR/mgR mice. Therefore, we hypothesize: 1) TAAD in MFS results primarily from KLF4 down-regulation and abnormal aortic SMC phenotypic switching from ED14 to PD14 in the mouse; 2) Similar SMC phenotypic changes occur normally in humans over a longer time span; 3) that increasing KLF4 levels during early aortic development will attenuate morphologic changes and prevent aneurysm formation in MFS. Four specific aims are proposed: 1) Confirm and better characterize aortic SMC phenotypic modulation in mgR/mgR mice during early development and correlate this with KLF4 expression; 2) In order to characterize SMC changes during the normal aortic development in humans and define a therapeutic window for MFS patients, we will study aortic SMC phenotype and KLF4 expression in infants and children of different ages; 3) Determine if early CsA treatment in mgR/mgR mice will increase KLF4 expression, alter SMC phenotype, and prevent aneurysm formation; 4) In order to verify that the CsA effects are mediated through KLF4, we will overexpress KLF4 in mgR/mgR mice and determine the effects on SMC phenotype and TAA formation/rupture. Successful execution of the experiments will provide novel insight into the initial pathogenesis of genetically predisposed TAAD and rupture observed in MFS. Our long-term goal is to identify the role of premature differentiation of aortic SMCs in MFS, to further understand the pathogenesis of other syndromic causes of thoracic aneurysms, and eventually to develop rational therapeutic strategies to prevent and treat TAA.